Purifying synthetic water-soluble carboxylic acids



Jan. 26, 1954 A. STEITZ, JR., ET AL PURIFYING SYNTHETIC WATER-SOLUBLE CARBOXYLIC ACIDS Filed Jan. 27,

Buf rl'c Acid ACET/C ACID COLUMN PROP/ON/C ACID COLUMN BATCH STILLS 2 Sheets-Sheet 1 Di/ufeMinem/ Acid \z a 9 INVENTORS: 9 q Alfred Sfeifz, Jr: QE Robe?!" HJacoby BY 4 TORNEY Patented Jan. 26, 1954 PURIFYING SYNTHETIC WATER-SOLUBLE CARBOXYLIC ACIDS Alfred Steitz, Jr., and Robert H. Jacoby, Tulsa, kla., assignors to Stanolind Oil and Gas Company, Tulsa, Okla., a corporation of Delaware Application January 27, 1949, Serial No. 73,144

Claims. 1

This invention relates to processes of purifying aliphatic acids, and more particularly the processes for removing complex organic contaminants and the like from acetic, propionic, butyric and higher aliphatic acids or mixtures thereof.

In the production of organic oxygenated compounds by various processes, such as the hydrogenation of carbon oxides, the oxidation of normally liquid hydrocarbons, action of micro-organisms, and the like processes, products of considerable complexity are obtained which comprise a wide variety of organic oxygenated compounds including acids, alcohols, aldehydes and ketones of a broad range of molecular weight. For example, when carbon monoxide is hydrogenated in the presence of an alkali-promoted fluidized finely divided iron catalyst at temperatures of between about 580 and 625 F. and under pressures of above 250 pounds per square inch, an aqueous phase and a liquid hydrocarbon phase are produced, both being rich in oxygenated compounds. The aqueous phase includes among other organic oxygenated compounds a recoverable proportion of' aliphatic acids such as acetic acid, propionic acid and butyric acids.

Simple, direct fractional distillation of either the hydrocarbon phase or the aqueous phase is not feasible because of the numerous multiple component azeotropes that are known to exist among the various constituents and because of a tendency of certain of the components to react, decompose or polymerize when such a mixture is exposed to elevated temperatures for suitable periods of time. In the recovery of oxygenated compounds from aqueous solutions it has been proposed to fractionallydistill the solutions to separate the oxygenated compounds into two groups, the alcohols, aldehydes and ketones being taken overhead and the acids being removed in aqueous solution at the bottom of the still. The top temperature of the fractionator is maintained at such a level that the overhead comprising predominantly alcohols, aldehydes and ketones is substantially free from acids and accordingly some of the heavier alcohols, aldehydes and ketones remain with the carboxylic acid fraction. These residual alcohols, aldehydes and ketones are removed with the acid fraction by extraction with ethyl acetate and the extract of ethyl acetate and mixed acids is subsequently distilled for the recovery and recycle of ethyl acetate to the extractor. The-mixed acids, accordingly, include some ethyl acetate and higher-boiling connate esters as the contaminant, as well as aldehydes water azeotrope.

and ketones produced in the synthesis. It is with resoect to this contaminated, mixed acid stream that our invention has particular refer-' ence.

The separation and purification of a crude mixture of acetic, propionic and butyricacids containing small amounts of contaminants present a difficult problem. The contaminants which are present in the acid mixtures apparently have boiling points near the boiling points of the acids or form azeotropic mi tures on distillation with the acids and the separation of uncontaminated acids, such as acetic, propionic or butyric acids, is made difiicult by the complex nature of the mixture in which they occur.. Thus, the acid stream to be purified may contain acetic, propionic and butyric acids and contaminants which may include esters, aldehydes and ketones corresponding to butyraldehyde and. ethyl acetate and higher.

Heretofore the contaminants which occurred with the aliphatic acids have been removed with considerable difliculty and at prohibitive expense. It is, therefore, a primary object of our invention to separate substantially pure aliphatic acids from contaminated mixtures thereof. Other objects and advanta es of our invention will become apparent from the following description taken with the drawings which form a part thereof.

We have discovered that the connate contaminants can be removed from the crude acids by distilling in the presence of a small-amount of added water to remove the contaminats as the The feed or acid stream supplied to the purification tower may, for example, be the water-free acid bottoms from the solventacid column in the extraction stream together with a sufiicient amount of added water to azeotrope with all of the contaminants. If desired,- a slight excess of water may be used to azeotrope a small proportion of the propionic and-butyricacids thereby assuring complete removal of contaminants. The water and the acid bottoms stream are introduced into a purification tower.

in the preferred embodiment, the streams areintroduced separately at vertically spaced points in the tower with the water entering below the acid. The overhead fraction from the acid purificationtower or zone can be cooled and separated into a water layer and a discard organic contaminant layer.

make-up water should be provided.

The separated water may be returned to the purifying tower to azeotrope with" additional contaminats, but means for adding It is desirable to have the slight excess of water so as to effect the substantially complete removal of the impurities as the water azeotropes of the contaminants. However, since the acetic acid must be free of water, the distillation may be continued so as to remove the residual excess water as, constant boiling mixtures of propionic and butyric acid with water. The bottoms from the tower will then be pure acetic acid, propionic acid and butyric and Valerie acids, except for very small amounts of very heavy impurities, such as iron, which cannot be removed. asan azeotrope. These latter heavy impurities may be removed as bottoms in the. distillation of the. butvric acid fraction.

The advantage of our invention includes the fact that a stream of pure anddry acids can be produced in a single fractionating tower and the acetic acid and propionic acid may be removed I from this pure, dry mixture in a subsequent tower leaving a butyric acid asbottoms. Alternatelyan; initial split canbe made between the propionic and butyric acids, the acetic; and prop-ionic acids being purified together and then separated.

Briefly, we may attain the'objects of ourinvention by removing the: impurities boiling below butyric acid 635 C.) "as the water azeotropein afirst column. Although waterdoes azeotrope with: small amounts of propionic and butyric acids; these azeotropes boil at atemperature higher than those ofthe'contaminant. Accordingly, it is possibleto-removeall the contaminants from the acids. Theoverhead from'the acid purification column,-which comprises thewater azeo tropes 'of the complex contaminants, is cooled and separated into; an organic layer and a waterlayer. The water layer is recycled tothe feed for: azeotroping with additional ouantities of the contaminantsand the organic layer may be dividedbetween reflux in the acid purification column andrecovery as product. The bottoms from the firstcolumn. is then introduced into a second column where.- acetic acid: is taken overhead. The propionic and butyric acids recovered as bottoms from-the second column areseparated underred-uced pressure ina third column where propionic acidis taken overhead; The nand iso-butyri'c acids obtained from. the third column as bottoms" are batch distilled under vacuum; The butyric acid iron salts remaining after such an operation are mixed with. water, acidified with dilute mineral. acid and the butyric" acid-water azeotrope distilled overhead in a final: batch-still and recycledas part of the feed: in the acid extraction system. If desired, the last step of. recovering thebu-tyric. acid from irornsalts may be. eliminated and. the bottoms fromthe. vacuum distillation discarded.

. Referring to Figure 1-. of: the drawing, which is a. diagrammatic illustration. of one embodiment ofour invention,- a mixed acid stream containing acetic, propionic and butyric: acid contaminated with. esters,;aldehydes and .ketones is introduced byline. Ill' into tower H which may be provided with tower top; cooling means and reboiler (not shown) Thefeed' contains: sufiicient water to azeotrope with the contaminants plus a'slight excess of water toazeotrope with small amounts of butyric and propionic-acids. Water is combined with the acid'stream via: line 12, and comprises predominantly water. which has been recycled as described, hereinafter, but make-up. water can be supplied by; line 1.3. It is also. contemplated that recycle or make-up water may be introduced ii rectly and separately to thetower ll.

Within the tower II, the water azeotropes o! the contaminants are taken overhead via line l4 at a temperature of about 210 F., passed through cooler I5 and introduced into the separator I6. In the separator It a lower water layer and an upper contaminant or organic layer separate. The water may be. recycled via, lines; I 2" and ill, or lines l2 and I2a, to azeotrope with additional quantities of organic contaminant within acid purification tower II. The organic layer separated in i6. i withdrawn via line I! and ordinarily will be withdrawn as product by line [9, but if desired a portion thereof may be returned as'reflux via line Hi. The reboiler temperature in tower II should be at the boiling point of the acetic-propionic acid mixture under the reboiler pressure so that no water will appear in the bottoms.

The purified, mixed aliphatic acids are withdrawn as bottoms via line 20 from tower II and introduced into tower 2 I. The mi-xedvdryacids; in line Zilcomprise acetic acid, propionic acid,- bu-a tyric-acid, and possibly va-leric acid, together with: a. small amount of iron salts. From the tower 2|, acetic acid is separatedfrom the-heavier acids andwithdrawn overhead via-line 22 at, atemperature. of about 245 and atmospheric pressure. passed. through cooler 23 and Withdrawn, as. concentrated acetic; acid productvia line: 2.4 with a. portion, being returned to tower 21 as reflux-via valved line25. The-reboiler temperature intower 2i shouldba the; boiling point of the acids heav-- ier than. acetic acid in order to insure that no. acetic acid is removed from the: bottom of, the column withtheother acids.

The bottoms from tower 2| are passed by line. 26-into-tower 2.!- wherein; propionic acid is: separated from; butyric acid and heavier components: under'reduced pressure. of between about 350 and about 485 mm. Hg, a tower top temperature of about 245 F., and a reboiler. temperature of about: 275 F. The propionic: acid product iswithdrawn overhead via, line, 28, passed through cooler 29 and. recovered via line 30. A portion of the.pr.o-- picnic acid product. may,,however, b returned. as reflux to the tower 21 via valved line 3L The bottoms from thepropionic .acidtower 21 comprise predominantly butyric 'a-nd valeric; acids and some. iron acidsalts- These bottoms: are supplied by line 32' to a reduced pressure batch still 33-. In this still- 3'3, butyric acid is recovered as an overhead fractionv byline 34 and the bottoms consist. essentially of iron acid salts;v These may be discarded, via line: 36' or treated in; batch. still 3! with, a; dilute mineral: acid such. as sulfuric acid introduced: via line 40: whereby the iron acid salts are hydrolized. to produce. a: dilute salt solution discarded; via, line.- Wand/ a; constant boiling. mixturerrof butyric acid. and wa ter which may be withdrawn overhead frombatch. still 3! by line, 39:. This constant. boiling mix;- ture. may be; recycled to the original: acid extrac tion step and :ultimatelywreturning to. tower H as part of the acid feedintroduced: byline ill.

The. embodiment: of our inventionillustrated in Figure 1 includes splitting the acid mixture into an acetic acid fraction: and a mixed propioni'c; butyric and Valerie fraction; it is contemplated that the split can be made between. propionic and butyric acids with a subsequent fractionation .ofxaceti'cand. propi'onic. This latter embodimentis-illustratedinEigure 2;

Reierringto Figure 2, which is a diagrammatic illustration of another embodiment of our invention, a contaminated crude" acid stream contaming acetic, 'propionic and heavier acids, such as butyric and valeric acids, is introduced by line 5| into acid splitter or fraetionator 52. Since there is likelihood of cracking the'butyric and heavier acids in tower 52 if too high a temperature is maintained therein, the tower is operated at a pressure below atmospheric. The maximum reboiler pressure allowable is about 500 mm. Hg in order to keep the reboiler temperature at or below about 300 F. These conditions fix the upper limit of the tower operating pressure and the lower limit of tower operating pressure depends upon the temperature of cooling water available. A heavy acid bottoms fraction, including butyric and valeric acids, is withdrawn by line 58 for further processing which may, for example, be as described in connection with line 32 et seq. of Figure 1. An overhead fraction comprising the lower-boiling acetic and propionic acids and their contaminants is withdrawn from the acid splitter '52 at a temperature of about 210 F. and passed by line 53 through condenser 54. A portion of the condensate may be refluxed to tower 52 via line 55 and the balance is passed by line 60 into the acid purification tower 6!.

Water is introduced into purification tower 6: via line 62 and comprises predominantly water which has been recycled from separator 65 as described hereinafter, but make-up water can be supplied by line t3 as needed. The amount of water introduced is enough to azeotrope with all the contaminants in the organic acid feed plus a slight excess. The use of excess water results in more complete purification of the acids, but also azeotropes with and carries acid overhead. Accordingly, the balance is between the extent of purification required and the proportion of acid lost which is allowable. For example, a ratio of between about 8 or 9 pounds of crude acids per pound ofwater teed gives satisfactory purification.

Within the tower 6!, the water azeotropes of the contaminants are taken overhead via line 64, and the tower is operated to produce an overhead vapor which can be condensed into two liquid phases. This overhead stream is passed through cooler 55 and introduced into separator 86. In the separator 85, a lower water layer and an upper contaminant or organic layer are separated. As much of the water or lower layer as necessary is returned to column ti via line 62 to azeotrope with additional quantities or organic contaminants entering column 6! with the crude acids in line 60. If desired, a portion of the entire condensate from cooler 65 may be returned as reflux to tower 6! via line 69. Ordinarily, however, a portion or" the water condensate from line 62 will be supplied as reflux via line 68 to the tower 5!. A small reflux ratio of between about 1 and 6 to 1 is preferred, the balance of the water being withdrawn from the system. If desired, an oxidizing agent, such as potassium permanganate, peracetic acid, or sodium hypochlorite, may be introduced into the acid purifier 6| as an aqueous solution via line 58 or an oxidizing agent may be introduced into the acetic acid column H via line 5? as a dry powder.

The purified, mixed aliphatic acids comprising acetic acid, propionic acid, and a small proportion of heavier acids together with some iron salts are withdrawn as bottoms via line it from tower 6i and introduced into tower N. This tower fractionates acetic acid from propionic acid. The top temperature may be maintained at about 245 F., the reboiler about 300 F., and

the outlet pressure at about atmospheric. Acetic acid is separated from the heavier acid fraction within the tower TI and withdrawn overhead via line 12, passed through cooler 13, and recovered as the acetic acid product via line M. A portion of the product may be returned to tower Tl as reflux via valved line 15.

The bottoms from the acetic acid column ii are introduced byline 16 into tower 11 wherein propionic acid is separated from butyric acid and heavier components under subatmospheric pressure. The propionic acid product stream is withdrawn overhead via line-18 at a temperature of about 245 F. and assed through condenser 19 for. recovery via line 80. A portion of the liquid propionic acid product may, however, be returned as reflux to the tower 'I'I via valved line 8|. The small streamof bottoms from the propionic acid column ll comprises predominantly butyric and some propionic acids. These acid bottoms may be withdrawn by line 82, or recycled via lines 55 and iii to the acid splitter 52 for recovery of butyric and heavier acids for further processing as described in connection with the heavier acids withdrawn via line 58. Alternatively, such processing may comprise, for example, extraction with water to separate organic water-insoluble contaminants therefrom, distillation with a hydrocarbon to dry the mixed acids, and fractionation under vacuum in the presence of an oxidizing agent to eliminate residual impurities.

The purposes of the purification of the mixed aliphatic acids are to decrease the turbidity upon subsequent dilution of the acid, to increase the permanganate time, and to increase the purity of the particular acid fractions. The permanganate time test consists of adding 0.1 ml. of 0.1 N potassium permanganate to 2 ml. of the acid under test dissolved in 10 ml. of water. The pink color should not disappear within two hours.

The main object of the removal of the organic contaminants which are separated as the water azeotrope is to meet the turbidity test. However, the mere removal of esters does not assure a good permanganate test and the real cause of the differences in permanganate time is not clear.

To demonstrate the effectiveness of this in vention in purifying crude acetic and propionic acid mixture produced by synthesis from hydrogen and carbon monoxide, a quantity of the mixture was distilled in the presence of 10% by volume of an aqueous solution containing 0.54% potassium permanganate. After removal of the water azeotropes by fractionation, dry acids were obtained having the following specifications:

Although we have described our invention with reference to preferred embodiments thereof, it will be apparent that modifications may be made in the details of our process without departing from the spirit thereof. For clarity, apparatus details such as pumps, reboilers, valves, and the like have been omitted from the drawing, but it is to be understood that the apparatus, techniques and procedures available to those skilled in the art may be used in adapting and applying our invention. It will be apparent, moreover, that while we prefer to effect the various operations of our process in a continuous manner, we

In general, it is intended that any modifications or equivalents of our process which might occur to one skilled in the art come within the scope of our invention. Furthermore, although the foregoing flow sheets and descriptions thereof illustrate advantageous embodiments of our invention, this is for the purpose of illustration only and we do not intend to be limited to the particular embodiments described.

What we claim is:

1. In a process for obtaining substantially pure dry acetic acid. and water-soluble carboxylic acid, which form minimum boiling azeotropes with water, from dilute. aqueous solutions thereof, said acids havingbeen produced by synthesis from hydrogen and carbon. monoxide and containing water-soluble organic oxygenated impurities simultaneouslyproduced with said acids, including water-soluble esters and carbonyl compounds which boil near the boiling points of said acids or form azeotropes with at least some or said acids in the absence of water, wherein a dilute aqueous mixture of said acids and impurities is first subjected to extraction with a substantially water-immiscible solvent for said acids, and the latter, together with said impurities, separated in'substantially anhydrous form from the resulting extract to obtain said acids in a substantially anhydrous fraction containing said impurities, the steps which comprise adding only'sufiicient water to said fraction to form minimum boiling azeotropes with said impurities, and thereafter subjecting the resulting mixture to distillation at a temperature below the boiling point of said first-mentioned minimum boiling azeotropes until said fraction is sufficiently free of said impurities to pass standard acid turbidity tests.

2. The process of claim 1 in which the acids present in said aqueous solution include propionic and butyric acids.

3. The process of claim 1 in which the substantially water-immiscible solvent for said acids is ethyl acetate.

4. In a process for obtainingsubstantially pure dry water-soluble carboxylic acids, which form. boiling azeotropes with. waters. from dilute aqueous. solutions thereof, said acidshaving been produced by synthesis from hydrogen and carbon. monoxide and. containing. watersoluble organic oxygenated impurities simultaneously produced with said acids, including. water-soluble esters and carbonyl compounds which boil near the boiling points of said acidsv or form azeotropes with at least some of said acids in the absence ofwater, wherein a dilute aqueous mixture-of s'aid acids and impurities isfirst subjected to extraction with a substantially water-immiscible solvent for said acids, and the latter, together with said impurities, separated in substantially anhydrous form from the resulting extract to obtain said acids in a substantial- 1y anhydrous fraction containing said impurities, the steps which comprise adding only sufficient water to said fraction to-form minimum boiling azeotropes with said impurities, and thereafter subjecting theresu'lti'ng mixture to distillation at a temperature below the boiling point of said firstmentioned minimum boiling azeotropes until said fraction is sufiiciently' free of said impurities to pass standard acid turbidity tests.

5. The process of claim 4 in which the acids present in said aqueous solution include propionic and butyric acids.

ALFRED STEITZ, JR.

ROBERT H. JACOBY.

References Cited in the file of this patent V UNITED STATES PATENTS I Number. Name Date 2,013,096 Haag Sept. 3, 1935 2,159,984 Fisher May 30, 1939 2,227,485 Bump Jan. 7, 1941 2,438,300 Schneipp Mar. 23, 1948 2,476,788 White July 19, 1949 2,533,675 Marschner Dec. 12, 1950 OTHER REFERENCES Rogers Manual of Industrial Chemistry, Sixth Edition, published by D. Van Nostrand Company, Inc., 250 Fourth Ave, New York 1942. Volv 1, pages 674-677. 

4. IN A PROCESS FOR OBTAINING SUBSTANTIALLY PURE DRY WATER-SOLUBLE CARBOXYLIC ACIDS, WHICH FORM MINIMUM BOILIN AZEOTROPES WITH WATER, FROM DULUTE AQUEOUS SOLUTIONS THEREOF, SAID ACIDS HAVING BEEN PRODUCED BY SYNTHESIS FROM HYDROGEN AND CARBON MONOXIDE AND CONTAINING WATERSOLUBLE ORGANIC OXYGENATED IMPURITIES SIMULTANEOUSLY PRODUCED WITH SAID ACIDS, INCLUDING WATER-SOLUBLE ESTERS AND CARBONYL COMPOUNDS WHICH BOIL NEAR THE BOILING POINTS OF SAID ACIDS OR FORM AZEOTROPES WITH AT LEAST SOME OF SAID ACIDS IN THE ABSENCE OF WATER, WHEREIN A DILUTE AQUEOUS MIXTURE OF SAID ACIDS AND IMPURITIES IS FIRST SUBJECTED TO EXTRACTION WITH A SUBSTANTIALLY WATER-IMMISCIBLE SOLVENT FOR SAID ACIDS, AND THE LATTER, TOGETHER WITH SAID IMPURITIES, SEPARATED IN SUBSTANTIALLY ANHYDROUS FORM FROM THE RESULTING EXTRACT TO OBTAIN SAID ACIDS IN AL SUBSTANTIALLY ANHYDROUS FRACTION CONTAINING SAID IMPURITIES, THE STEPS WHICH COMPRISES ADDING ONLY SUFFICIENT WATER TO SAID FRACTION TO FORM MINIMUM BOILING AZEOTROPES WITH SAID IMPURITIES, AND THERAFTER SUBJECTING THE RESULTING MIXTURE TO DISTILLATION AT A TEMPERATURE BELOW THE BOILING POINT OF SAID FIRSTMENTIONED MINIMUM BOILING AZEOTROPES UNTIL SAID FRACTION IS SUFFICIENTLY FREE OF SAID IMPURITIES TO PASS STANDARD ACID TURBIDITY TESTS. 